Large areas of North America, western Europe and Australia/New Zealand produce crops that are deficient in selenium, copper and other essential trace elements. The absence or deficiency of these trace elements affects the overall food chain, resulting in plants, and ultimately animals, deficient in these minerals. Significant deficiencies in any or a combination of specific trace elements will affect the overall health of many animals and may lead to a variety of acute health problems. Accordingly, a correct balance of trace elements is required for optimal health and livestock production. In particular, selenium is a trace element required by animals for a variety of biological processes. It is essential that animals receive appropriate levels of selenium. In all classes of livestock (dairy, beef, swine and poultry) selenium deficiency causes a variety of chronic and acute health problems.
Numerous animal nutritionists have worked on the problem of selenium deficiency in animals for years. While crops are not in themselves affected by low levels of selenium in the soil, the use of crops containing almost no selenium as animal fodder or feedstuffs leads to selenium deficiencies in all classes of animals, including humans.
Numerous methods have evolved to address the problem of selenium deficiency, which have certain problems and limitations associated with them.
Selenium is routinely provided to livestock in mineral blocks (referred to as free choice), injections and/or as mineral supplements mixed with feed.
With free choice supplements, the amount of selenium each animal intakes is not controllable and varies widely because some animals over self-administer and others under self-administer. Therefore, some of the herd/flock receive inadequate amounts and others excessive amounts of selenium (Hemingway, R. G. 1982).
Injections of selenium are time consuming, costly, need to be repeated, and involve additional handling of animals which is dangerous to the handler and stressful to the animal.
Using off-farm premixed feeds containing selenium is often not appropriate since many farmers use mostly home-grown feed and do not purchase feed from suppliers. These farmers would have to mix selenium into their farm-grown feed, which is problematic since many farms lack suitable mixing facilities and handling selenium is potentially dangerous. In cases where pre-mixed concentrated feeds are purchased, the amount of selenium required and the amount of concentrated feed used varies with each farm.
Thus, custom selenium supplementation for each farm may be required, adding to the complexity. These variables coupled with the fact that additional handling of the feed at the feed plant would be required to mix in the selenium would make the pre-mixed concentrate feeds more expensive. Therefore, pre-mixed feeds are not an ideal solution to the selenium deficiency problem.
There are a significant number of hectares of corn and other fodder crops grown in selenium deficient areas. Corn is the most important feed grain in the United States for all classes of livestock because of its high quality and high yields. Silage corn is grown as whole crop feeds on many dairy and beef farms. Corn and cereal silage are produced in Canada. Farm grown corn, pulse crops and cereal grains are also important feeds for the swine and poultry industry.
Several corn growing areas are generally deficient in selenium including, for example, Maritime provinces, Quebec, Ontario, parts of British Columbia, New England, Illinois, Indiana, Michigan, New York, North Carolina, Ohio, Pennsylvania and Wisconsin. These areas are also important dairy producers. In cooler areas, cereals replace corn in selenium deficient areas.
While it has been shown experimentally that selenium can be effectively applied by spraying a selenium solution onto plants (Gupta et al 1988), this practice is not in commercial use in view of the numerous limitations associated with plant spraying. In particular, spraying plant foliage with a selenium solution is inconvenient because it requires an additional pass over the field with a carefully prepared solution and expensive calibrated spraying equipment. Timing of the spraying is very important; spraying cannot be done in windy weather and rain will wash off the spray. Also plants must be at a sufficiently advanced growth stage to ensure uptake but applying at a stage of over advanced growth will result in mechanical damage to the crop from the tractor and application equipment.
The only commercial method now in use anywhere in the world for enhancing selenium in crops grown on selenium deficient soils is by application of a slow-release prilled selenium fertilizer on grasslands usually by aircraft. Selenium applied to the soil in this way is taken up less efficiently than selenium delivered via the seed coating method. This method is used primarily in Australia and New Zealand and has rarely been used in North America to date on a commercial scale.
Application of selenium as a fertilizer is impractical for the following reasons:
i) the selenium fertilizer would have to be in a prilled form since powdered selenium does not have the physical characteristics required for proper handling and blending, PA1 ii) prilled selenium fertilizer cannot be applied with conventional farm equipment because the application rates are far too low. That is, conventional farm equipment cannot be slowed down or regulated enough to accurately apply the selenium with the high degree of accuracy that is required. The lowest rate of fertilizer this equipment can accurately apply is 20 kg/ha whereas prilled selenium fertilizer must be applied at a rate of 1 kg/ha, PA1 iii) blending selenium with other fertilizers requires the use of prilled selenium fertilizer; PA1 iv) it is practically impossible for a farmer to blend prilled selenium with other fertilizer on their farm since this requires the proper mixing equipment (which farmers do not have) to ensure that exact minute amounts of selenium are uniformly blended throughout large volumes of fertilizers; PA1 v) the only forms of prilled selenium fertilizer that are currently commercially available are slow-release which is unsuitable for certain plants such as corn since the selenium is not rapidly available in the soil; PA1 vi) the blending process is potentially dangerous to the farmer because of the physical handling and the potential of inhaling air-borne selenium dust or the potential of skin contact or absorption of the selenium dust; PA1 vii) applying selenium blended with fertilizer is impractical because all fields require different amounts and types of fertilizer which would require custom blending for every field which would be difficult and expensive; PA1 viii) blending the prilled selenium with an inert dilutant (eg vermiculite) would add to the cost to fertilize the field. PA1 a) solubilizing a selenium-containing compound in water to form an aqueous selenium solution; PA1 b) mixing the aqueous selenium solution with a water-soluble polymer emulsion to form a polymer/selenium solution; PA1 c) mixing the polymer/selenium solution with a sufficient quantity of seeds to form a film coat on the seeds; PA1 d) curing the seeds from step c) to form an abrasion-resistant protective coating on the seeds.
There has been a need to deliver selenium to crops via a seed coating technology that overcomes the aforementioned limitations and other problems associated with delivering the selenium with fertilizers.
Seeding rates for individual crops are relatively uniform over wide geographic areas, usually varying by less than 20% in contrast to fertilizer rates which vary greatly (0-500 kg/ha of product). Delivering selenium to the crop via the seed, rather than fertilizer, is better since a controlled level of selenium in the seed coat provides a more predictable and optimal amount of selenium in any given crop. Further, it is easier for the farmer to assess whether the correct amount of selenium has been applied as rate of application is related to emerging plant population which can be checked visibly. Such checks cannot be done with fertilizer application rates. Further, fertilizer is not suitable to deliver selenium because fertilizer needs vary so significantly from field to field and crop to crop.
Custom blending selenium whether at a fertilizer plant or on a farm can easily lead to errors in selenium application rates. Low rates will lead to the health problems livestock experience with selenium deficiency and excessive rates may threaten animal health, plant health or the environment.
A coated seed simplifies these quality control issues since the exact amount of selenium applied to each seed at the seed-coating plant can be very accurately controlled.
As the exact amount of selenium can be put on each seed and the plant uptakes the selenium at a predictable and consistent rate, it is far easier to control the exact amount of selenium in the crop. Other methods do not allow for this level of control. Seed coating allows for the control, consistency and repeatability from year to year that is required for proper livestock management practices.
Accordingly, there has been a need for a technology that overcomes the aforementioned problems and limitations. Specifically, there has been a need for a technology that effectively raises selenium levels in crops in an efficient, uniform, convenient, consistent, safe manner that also allows for accurate, consistent manipulation of the level of selenium in the crop. Seed coating technology can achieve this goal. Gissel-Nielsen et al (1984) disclose the application of selenium solutions onto seeds (pages 424-425) with variable results and suggest that seed coating technologies may be used for the administration of selenium to the seed. However, no such technologies are disclosed, nor were any coated seeds produced.
In summary, there has been a need for selenium seed coating technologies that assures the farmer that his crop will contain enough selenium every year, regardless of geographical location, without additional work or inputs and that specifically address the following criteria:
1. Toxicity
Selenium is potentially toxic to both plants and animals above certain levels and, accordingly, must be handled as such.
a) Phytotoxicity
With respect to plants, the primary issue is phytotoxicity, Selenium applied directly to plants above certain levels, may result in reduced plant growth or possibly kill the plant. In the past, it has also been considered that the application of selenium directly to the seed of the plant would have a phytotoxic effect in view of the relatively high concentration of selenium immediately adjacent the new plant roots during germination. This was considered to be of particular concern especially as the levels of selenium are increased. Previous studies have shown that selenium can negatively affect seed germination and elongation of young roots in several crop species (Levine 1925, Spencer and Siegel 1978, Carlson et al. 1989). In the study by Carlson et al (1989) the length of the young roots of sorghum, which is related to corn, were reduced by concentrations of selenium of 16-32 mg/litre of solution. In another study, selenium reduced yield of sorghum by up to 95% (Carlson et al. 1991). In this trial, sodium selenate was more deleterious to sorghum than sodium selenite. Wheat and barley may be more resistant to selenium than sorghum (Carlson et al. 1989, Ylaranta 1983).
b) Human Toxicity
With respect to humans, the primary issue is the toxicity associated with the inhalation of air-borne selenium or absorption of the selenium through the skin. Past experimental seed coating techniques using selenium, such as that disclosed in Gupta et al (1983) simply involved a process where the seed shell is made sticky with gum arabic and peat moss to which a slurry of water and selenium is added. The seed was dried leaving a certain amount of selenium attached to the seed. Handling of the seed leads to two problems. The first is the loss of the selenium coat from the seed by abrasion thereby leaving an inconsistent amount of selenium on each seed resulting in inconsistent and unpredictable levels of selenium in each plant and therefore leading to unpredictable levels of selenium in the feed. The second problem is the danger of inhaling air-borne selenium produced through handling the seed or prilled fertilizer or the absorption of the selenium through the skin when workers handle the seed or prilled fertilizer.
2. Linear Rate Response and Repeatability from Year to Year
Uptake of selenium by plant roots is regulated in part at least by energy requiring processes (Arvy 1993). Accordingly, the efficiency of selenium uptake depends on the energy status of the plant and growing conditions would be expected to affect selenium content. Selenium uptake is also affected by sulphur content of the soil. Some plant species are adapted to actively exclude selenium (Wu and Huang 1992). Based on these factors, the rate of uptake should vary within the year and from year to year. Indeed, Gupta and Macleod (1994) reported that soybean cv. Maple Isle receiving 10 g/ha of selenium (as selenate) contained 599 ppb in 1989 and 1458 ppb in 1990 even though the two sites had similar soil characteristics. Unpredictable levels of selenium uptake by the plant may lead to either deficient or toxic levels of selenium in the animal feed. Therefore, any commercially acceptable solution to the selenium deficiency problem has to overcome this as good livestock management practices requires that a farmer can accurately and consistently control the amount of selenium in the feed from year to year.
3. Efficient Uptake
Recovery rates for selenium applied (as selenate) to barley in the soil was at a rate of 4-10% (Ylaranta 1983). Similar rates of uptake efficiency occur with wheat utilizing a variety of application methods (Stephen et al 1989). It has been demonstrated that corn takes up about 5% of soil supplied selenium. Commercial available prilled selenium designed for slow release, has been shown to be taken up with even less efficiency, Accordingly, a technology is required to improve the uptake efficiency of selenium to reduce loss and risk of contamination to the environment.
4. Distribution of Selenium in the Plant
Corn is fed to ruminant animals either as a whole plant or as grain whereas non-ruminants, such as pigs and fowl, use only the grain portion of the corn or other crops. Thus for monograstrics it is important that the selenium is translocated to the cobs and not just stored in the leaves. Previous work has shown a greater concentration of selenium in the grain compared to straw in barley and soybeans (Ylaranta 1983, Gupta and Macleod 1994, Carey and Allaway 1973). Uptake of selenium from a seed coat was expected to be more rapid than soil applied selenium because of the proximity of the new roots to the selenium. Since selenate is rapidly absorbed and translocated in the plant, it was expected that most of the selenate would be quickly stored in the young leaves.
5. Optimum Selenium Formulation
It is known that particular forms of selenium do not work on particular plants. For example, selenium, as barium selenate, is not effective with corn in its uptake whereas for grass, uptake is effective. Accordingly, an optimal selenium formulation is required to ensure efficient uptake.
6. Uniform Application on Seed
Uneven application of selenium to seed results in some plants receiving greater selenium rates than others. Accordingly, because selenium is potentially phytotoxic, uniform application of selenium to each seed is necessary to avoid some seeds receiving toxic levels of selenium.
In view of the problems of producing selenium enhanced plants without the associated risk of plant, human or animal toxicity, there has been a need for a convenient, effective method for addressing the problems associated with livestock production in selenium deficient areas. Specifically, there has been a need for a convenient, effective method of ensuring plant uptake of selenium to produce plants which provide the required levels of selenium in animal feed in a consistent, reliable manner to enhance livestock vitality while ensuring that the plants, animals and handlers are not subjected to toxic levels of selenium.
Furthermore, there has also been a need for a method of supplying selenium to animals through the animal feed in order to eliminate the need for secondary treatment of the animals (injections) while ensuring a constant and consistent supply of selenium in the animals' diet.
Furthermore and more specifically, there has been a need for a seed coating technique in which a seed is coated with a seed coating containing selenium that accurately places an exact and uniform amount of selenium on each seed and thereafter covered by a water-soluble polymer film that encapsulates the entire seed. There has also been a need for a seed coating technique where the polymer coating is sufficiently resilient to prevent dusting off so dust borne levels of selenium do not rise to toxic levels during normal handling of the seed and to prevent each seed from ending up with non-uniform amount of selenium before planting. There has also been a need for a seed coating technique where a nutrient, fungicide and/or biocide coat may be optionally provided between the seed treatment coat and the polymer film to enhance the early growth of the seed. There has also been a need for coated seeds which improve the flow of seeds during handling.
Seeds of certain crops, such as corn, are routinely treated with fungicides. Accordingly, there has also been a need for a selenium seed-coating technology that incorporates fungicides into the seed coats wherein the seed coats also provide a protective layer thus rendering the fungicide/selenium treated seed safer to handle.
Past seed coating techniques have generally addressed the problems associated with improving the growth of plants and have not addressed the problem of enhancing animal fodder. Examples of soil and seed coating or treatment techniques are found in the prior art. For example, U.S. Pat. No. 4,735,015 discloses a seed protective coating using a film forming composition comprising polyoxyethylene-polyoxybutylene block co-polymers for controlling water uptake by the seed.
U.S. Pat. No. 3,911,183 discloses a seed coating process in which a seed is coated with polymer-pesticide film. Halogenated vinyl resin is used as a film former and pesticide carrier.
U.S. Pat. No. 3,698,133 discloses a plant seed with a multiple coating. The coating is in two layers, an inner porous coating permeable to water and an outer coating of a polymer with a controlled permeability to water. This patent discloses the use of additives to enhance particular functions of the plant.
U.S. Pat. No. 4,735,017 discloses a coated seed having inorganic additives in the coating.
U.S. Pat. No. 5,017,374 discloses a seed dressing which affords trace elements, such as Zn and Cr to livestock by plant uptake from the coated seeds using fungal spores.
U.S. Pat. No. 5,044,116 discloses a method for polymer coating of seed, which coating may include additives.
U.S. Pat. No. 4,229,613 discloses compositions for the controlled release of trace nutrients into soil including selenium.
Canadian Patent Application 2,042,661 discloses a selenium composition for application to soil and pastures. The composition includes barium selenate in a slow release form and water-soluble selenium compound.
U.S. Pat. No. 4,388,303 discloses the use of selenium (as selenite absorbed soil) in a plant for repelling foragers by inducing nausea.
U.S. Pat. No. 4,656,083 discloses the conversion of selenium metal to odiferous dimethyl selenide by soil bearing selenium-metabolizing organisms for repelling foragers.
U.S. Pat. Nos. 4,847,087 and 5,169,647 disclose adding selenium from selenium-in-sulfur compositions to soil for plant uptake so that ruminant animals may receive nutrient selenium.
U.S. Pat. No. 4,880,628 discloses a sustained release composition for livestock having a trace element, for example selenium, in a slowly soluble cement.
U.S. Pat. No. 4,251,952 discloses the use of water-insoluble polymeric materials blended with sugar for coating seeds. Although there is suggestion within this application that such seed-coating compositions may be used for the application of a range of compounds, including micronutrients, the use of selenium was not disclosed or contemplated in any manner. Furthermore, the present invention is directed to the use of water-soluble polymers v. insoluble polymers.
During development of a selenium-coated seed it was observed that many commercially available polymers are not compatible with selenium. The addition of selenium to the polymeric materials lead to coagulation or precipitation, or the selenium-polymer mixture was generally immiscible. Following screening of many water-soluble polymeric materials, several classes of polymers were found to be compatible with selenium and useful for application of high rates of selenium to seeds.